Redundant array of independent disks (RAID) and redundant array of independent tapes (RAIT) are data storage schemes that manage a bank of physical disk or tape drives as an orderly array such that the bank appears to be one logical disk or tape drive. A RAID or RAIT system functions as an array which all drives of the array must logically advance at the same rate as data is written across the array in stripes. The implementation of a RAID or RAIT may be redundant, however, without redundancy the loss of even one drive is catastrophic for all transactions (past and future).
RAID/RAIT systems are both rigid architectures. The size of the array (number of data drives, number of parity drives), size of the data stripes, etc., all are determined when the RAID/RAIT is initialized and remain constant throughout the life of the system. This architecture creates some inefficiency in file size, drive management, and failure recovery flexibility. The inefficiency in file size is caused by the definition of the data stripe. If the data to be written is not an even multiple of the strip size, then the data will be padded out in the array with NULL blocks (a waste of space and time). The inefficiency of drive management is caused by the systems attempt to complete one stripe across all the drives before proceeding onto the next stripe. This is not very noticeable in a RAID because maximum access time to any point on a disk is very small. However, it is very noticeable in a RAIT because a RAIT can not adapt to the unavailability (even momentarily) of a drive, or to greatly changing data rates.
Tape drives in a RAIT suffer from internal maintenance problems (like tape re-tensioning) that don't apply to disk drives of a RAID. As a result, when one tape in the RAIT pauses to perform an internal maintenance task, the entire RAIT pauses. Tapes are also affected by slow data rates more than disks. If the data is not ready when the tape is ready, the tape drive must stop, then when the data is ready, the drive must re-start, and possibly rewind and re-acquire its write location. This continual starting and stopping (thrashing) causes excessive wear on the equipment and wastes a lot of time. For a RAIT to avoid thrashing, it must maintain a minimum data rate greater that the sum of the minimum write speed for all the tape drives in the RAIT. RAID/RAIT systems are also not very flexible since the number of active data drives is fixed and the system can not operate when it is missing even one data drive from the defined minimum number of drives.
The minimum and maximum data rates of a RAIT system suffers from the above-described aspects of the RAIT architecture. Moreover, because of overhead associated with the RAIT system structure, the maximum data rate of a RAIT system never approximates the sum of the maximum data rates of the individual drives, and in practice is often less than half the sum of the maximum data rates of the individual drives. For example, in one conventional configuration, a specialized controller with a RAITO configuration of 4 LTO-3 tape drives (each capable of recording natively 20-80 megabits per second) can only operate at a system rate or 260 gigabits/hour, as compared to the 1.1 terabit per hour capacity that represents the sum of the native recording capacity of the 4 individual LTO-3 drives.